Nanoparticles are of great scientific interest as they can be utilized in many industrial or medical applications. Nanoparticles are typically sized between 1 to 100 nm.
In particular, gold nanoparticles have been intensively studied as they are versatile materials having interesting chemical, electronic and optical properties for a broad range of different applications. The properties and applications of gold nanoparticles, especially of porous gold oxide nanoparticles strongly depend on their respective shape and size.
Possible applications of gold nanoparticles and porous gold oxide nanoparticles lie, for example, in the fields of nanoelectronics, imaging, sensing, catalysis, optics, environmental industry, energy development and biomedicine. Due to the low metal oxidation potential of gold nanoparticles, they can be used in medical diagnostic tests, such as labeling, X-ray contrasting, immunestrain and phago kinetic tracking studies, in targeted drug delivery techniques, as well as in medical therapies.
Silver nanoparticles and porous silver oxide nanoparticles have various and important applications. Historically, silver has been known to have a disinfecting effect and has been found in applications ranging from traditional medicines to culinary items. It has been reported that silver nanoparticles (AgNPs) and silver oxide nanoparticles (AgONPs) are non-toxic to humans and most effective against bacteria, virus and other eukaryotic micro-organism at low concentrations and without any side effects. Moreover, several salts of silver and their derivatives are commercially manufactured as antimicrobial agents. In small concentrations, silver is safe for human cells, but lethal for microorganisms. Antimicrobial capability of AgNPs and AgONPs allows them to be suitably employed in numerous household products such as textiles, as well as disinfection in water treatment, food storage containers, home appliances and in medical devices. The most important application of silver, AgNPs and AgONPs is in medical industry such as topical ointments to prevent infection against burn and open wounds.
Several methods for producing porous noble metal oxide nanoparticles have been developed which utilize harsh conditions. Wet methods often require the application of aggressive reducing agents, for example sodium borhydride, capping agents and may additionally need organic solvents such as toluene or chloroform. Furthermore, often toxic compounds must be employed or are produced during the synthesis of porous noble metal oxide nanoparticles. Although known methods may produce successfully porous noble metal oxide nanoparticles, energy preparation consumption and pollution effects are relatively high, as well as material and environmental costs. Even the availability of some materials, in particular of biomaterials, as for example plant materials, may be a problem. In consequence, there remains a need for more cost-effective and environmentally benign alternative methods for producing porous noble metal oxide nanoparticles with improved properties on a large scale. Main criteria for a green chemistry synthesis of stabilized nanoparticles are the choice of eco-friendly and non-hazardous solvents, reducing agents and capping agents, especially for porous noble metal oxide nanoparticles which shall be utilized in medical treatment.
Biological synthesis of nanoparticles by plant extracts is at present under exploitation as some researchers worked on it and tested them for antimicrobial activities.
Chemical reduction methods are widely used for synthesizing AgONPs because of their readiness to generate AgONPs under gentle conditions and their ability to synthesize AgONPs on a large scale.
U.S. Pat. No. 5,540,834 discloses methods for the preparation of porous inorganic particles, preferably ZrO2 particles, that have a porosity of about 5-60%. The method involves the step of combining an aqueous sol comprising a colloidal dispersion of inorganic particles with a polymerizable organic material, polymerizing the organic material and forming aggregates over the polymer and inorganic colloidal particles, collecting the aggregates and pretreating them in a generally oxygen-free atmosphere, pyrolyzing the pretreated aggregates at temperature of less than 550° C. in an oxygen atmosphere and sintering the substantially polymer-free particles.
U.S. Pat. No. 5,182,016 relates to polymer-coated carbon-clad inorganic oxide particles, preferably ZrO2, which are useful in sorbent applications. The carbon-clad ZrO2 particles are preferably prepared by a low pressure chemical vapor deposition (CVD) method.
U.S. Pat. No. 7,276,224 B2 discloses methods of producing nanoporous particles by spray pyrolysis of a precursor composition including a reactive precursor salt and a nonreactive matrix salt, wherein the matrix salt is used as a templating medium. By this method nanoporous aluminum oxide particles which have a pore size of at least about 2 nm and no greater than 25 nm are provided.
X. L. Zhai et al., Chin. Chem. Lett., 15, 1342-1344 (2004) reports on porous carrier MgO which was aggregated by nanoparticles that have been firstly prepared by using a normal technology route. The MgO was rod-shaped and had large surface area.
Z. Lu et al., Int. J. Electrochem. Sci., 8, 3564-3571 (2013) describes Ag—Zn alloy that are dealloyed in 0.1M H2SO4 at a low temperature to fabricate nanoporous silver. The dealloying process involves the dissolution of the less noble element and a formation/coarsening of the nanoporous structure by surface diffusion of the more noble element. The formation of nanoporous structure is a process of selective dissolution of zinc atoms and diffusion of gold atoms at alloy/electrolyte interfaces.
For the last two decades extensive work has been done to develop new drugs from natural products because of the resistance of microorganisms to the existing drugs. Nature has been an important source of products currently being used in medical practice.
There are various strategies for using gold nanoparticles as a drug delivery vehicle, including systems based on covalent binding or drug encapsulation. Furthermore, it has been reported that antibiotics often disturb the bacterial flora of digestive tract which may develop multiple drug-resistant isolates, hence noble ways of formulating biocide materials is an upcoming field of attraction. For this reason, there is a need for the use of an agent which does not generate resistance and presents a good bactericidal property. Gold nanoparticles have a great bactericidal effect on several ranges of microorganisms.
A number of synthetic methods have been employed for the synthesis of silver-based nanoparticles involving physical, chemical and biochemical techniques. However, these chemical synthesis methods employ toxic chemicals in the synthesis route which may have adverse effect in the medical applications and hazard to environment.